1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly to transmissive liquid crystal display devices using a cholesteric liquid crystal polarizing plate and a cholesteric liquid crystal color filter layer.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device makes use of optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational alignment that results from their thin and long shape. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field to the liquid crystal molecules. Accordingly, as an intensity of the applied electric field changes, the alignment orientation of the liquid crystal molecules also changes. Since incident light through a liquid crystal material is refracted due to an orientation of the liquid crystal molecules resulting from the optical anisotropy of the aligned liquid crystal molecules, an intensity of the incident light can be controlled and images can be displayed.
Among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices, in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are disposed in matrix, have been developed because of their high resolution and superior display of moving images.
FIG. 1 is a schematic perspective view of a liquid crystal display device according to the related art.
In FIG. 1, the liquid crystal display (LCD) device 11 includes upper and lower substrates 5 and 22, and a liquid crystal layer 14 interposed therebetween. A black matrix 6 and a color filter layer 8 including red, green and blue sub color filters 8a, 8b and 8c are formed on the upper substrate 5. A transparent common electrode 18 is formed on the color filter layer 8 and the black matrix 6. The upper substrate 5 is referred to as a color filter substrate. A pixel electrode 17 of a pixel region “P,” a switching element “T” and array lines including a gate line 13 and a data line 15 are formed on the lower substrate 22. The lower substrate 22 is referred to as an array substrate. The switching element “T” is disposed in matrix and connected to the gate line 13 and the data line 15. The pixel region “P” is defined by crossing of the gate lines 13 and the data lines 15. The pixel electrode 17 at the pixel region “P” is made of transparent conductive material such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO) having high transmittance. A backlight unit 30 is disposed under the LCD device 11 as a light source.
When a gate signal is applied to the switching element “T,” a data signal is applied to the pixel electrode 17. When a gate signal is not applied to the switching element “T,” a data signal cannot be applied to the pixel electrode 17. That is, the LCD device 11 is a kind of light modulating device using light emitted from the backlight unit 30. Since the light from the backlight unit 30 passes through a plurality of optical films to display images, the LCD device 11 has a poor light efficiency. The plurality of optical films include a pair of linear polarizing plates (not shown) and a color filter layer 8. The pair of linear polarizing plates transmits only linear components of the light from the backlight unit 30. That is, the pair of polarizing plates transmits only linearly polarized light along a specific direction. Accordingly, only a portion less than about half of the light emitted from the backlight unit 30 passes through the pair of linear polarizing plates. The backlight unit 30 is not efficiently used, thereby a brightness of the LCD device reduced. Moreover, the color filter layer 8 of an absorption type causes heavy losses of the light from the backlight unit 30 and reduction of brightness. To solve the problem of brightness reduction, the color filter layer 30 should be formed to have high transmittance. However, high transmittance of the color filter layer 30 is obtained with reduction of color purity. Accordingly, there is a limitation to increase transmittance of the absorption type color filter layer 30.
To solve the brightness problem of LCD devices using an absorption type color filter layer, LCD devices using a cholesteric liquid crystal color filter (CCF) layer have been researched and developed. The CCF layer uses a selective reflection property of cholesteric liquid crystal (CLC). Since a wavelength band of transmitted or reflected light is determined according to a helical pitch of the CLC, a CCF layer can be obtained by forming a CLC to have a different helical pitch according to a pixel region. Contrary to an absorption type color filter layer, the CCF layer uses a selective reflection property. Accordingly, a light efficiency is improved by reducing losses of the light from the backlight unit.
FIG. 2 is a schematic cross-sectional view of a transmissive liquid crystal display device using a cholesteric liquid crystal color filter layer according to the related art.
In FIG. 2, a transmissive liquid crystal display (LCD) device 50 includes first and second substrates 52 and 58 facing into and spaced apart from each other. A cholesteric liquid crystal color filter (CCF) layer 54 is formed on an inner surface of the first substrate 52 and a cholesteric liquid crystal (CLC) polarizing layer 64 is formed on an outer surface of the first substrate 52. A retardation layer 60 such as a quarter weave plate (QWP: λ/4 plate) and a linear polarizing layer 62 are sequentially formed on an outer surface of the second substrate 58. A liquid crystal layer 56 is formed between the CCF layer 54 and an inner surface of the second substrate 58. A backlight unit 66 is formed outside the CLC polarizing layer 64.
In cholesteric liquid crystal (CLC) used for the CCF layer 54 and the CLC polarizing layer 64, alignment vectors of CLC molecules form a helical structure. The CLC molecules twisted with a right-handed helical direction reflect only right-handed circularly polarized light, while the CLC molecules twisted with a left-handed helical direction reflect only left-handed circularly polarized light. When incident light has a polarization state such that the circular polarization direction is the same as the helical direction and satisfies a Bragg's reflection condition, the incident light is reflected. For example, the CCF layer 54 has a left-handed helical direction and the CLC polarizing layer 64 has a right-handed helical direction. Accordingly, only left-handed circularly polarized light of incident light passes through the CLC polarizing layer 64. The left-handed circularly polarized light also passes through the CCF layer 54 and reaches the liquid crystal layer 56. The CCF layer 54 is formed to display one of red (R), green (G) and blue (B) colors in each pixel region. For example, in a pixel region for red color, the CCF layer 54 is formed to have helical pitches corresponding to green and blue colors. Thus, left-handed circularly polarized light corresponding to green and blue colors is reflected at the CCF layer 54 and only left-handed circularly polarized light corresponding to red color passes through the CCF layer 54. Similar formation of the CCF layer 54 can be applied to pixel regions for green and blue colors.
FIG. 3 is a schematic cross-sectional view illustrating polarization state of light passing through a transmissive liquid crystal display device according to the related art.
In FIG. 3, non-polarized light emitted from a backlight unit 66 includes nearly all wavelengths to have a broadband of wavelength. Among the non-polarized light, right-handed circularly polarized light reflects from a CLC polarizing layer 64 to the backlight unit 66 and only left-handed circularly polarized light passes through the CLC polarizing layer 64 according to characteristics of the CLC polarizing layer 64. When the right-handed circularly polarized light reflecting from the CLC polarizing layer 64 again reflects from the backlight unit 66, the circular polarization direction is inverted such that the right-handed circularly polarized light becomes left-handed circularly polarized light. Accordingly, the left-handed circularly polarized light reflecting from the backlight unit 66 can pass through the CLC polarizing layer 64. Therefore, most circularly polarized light finally has left-handedness during a recycling process of light and passes through the CLC polarizing layer 64.
When the left-handed circularly polarized light having a broadband of wavelength meets a CCF layer 54 of one pixel region, left-handed circularly polarized light having a wavelength corresponding to one of red, green and blue colors passes through the CCF layer 54. For example, in a pixel region for red color, the CCF layer 54 is formed to have a first CLC layer (not shown) reflecting only left-handed circularly polarized light having a wavelength corresponding to green color and a second CLC layer (not shown) reflecting only left-handed circularly polarized light having a wavelength corresponding to blue color. Accordingly, left-handed circularly polarized light having a wavelength corresponding to red color can pass through the CCF layer 54 in a pixel region for red color. When the left-handed circularly polarized light reflecting from the CCF layer 54 again reflects from the CLC polarizing layer 64, the circular polarization direction is inverted such that the left-handed circularly polarized light becomes right-handed circularly polarized light. Accordingly, the right-handed circularly polarized light reflecting from the CLC polarizing layer 64 can pass through the CCF layer 54. By repetition of the aforementioned process, most light having a wavelength corresponding to a specific color can pass the CCF layer 54 without loss.
While the circularly polarized light that has passed through the CCF layer 54 passes a liquid crystal layer 56 and a retardation layer 60, the circularly polarized light is retarded to be a linearly polarized light having a polarization direction parallel to an optical axis of a linear polarizing layer 62 and then emitted to the outside.
Since the CLC molecules have a property of recycling light, the CLC polarizing layer and the CCF layer have higher transmittance than a conventional linear polarizing layer and an absorption type color filter layer. Accordingly, high brightness can be obtained. However, reflected light for an obliquely incident light has a shorter wavelength than that for a perpendicularly incident light. As a result, light obliquely emitted from an LCD panel has different color (i.e. wavelength) from light perpendicularly emitted from the LCD panel. This difference causes a color inversion and a narrow viewing angle.